The present invention relates generally to a measuring circuit for measuring an AC voltage. More particularly, the invention is concerned with a measuring circuit which is capable of measuring a three-phase AC power supply with high precision or accuracy while eliminating induction noise notwithstanding high impedance.
The conventional measuring circuit known heretofore is constituted by a voltage source whose impedance is as low as possible and designed such that a signal circuit to be shielded is covered with a shielding circuit whose electrical potential changes similarly or equally to a specific voltage behavior of the signal circuit to be shielded or is fixed stably. However, in a signal circuit in which the voltage changes over a wide range as in the case of a high-resistance bleeder, the shielding potential can not definitely be specified with the conventional method. Besides, the influence of parasitic capacitance attributable to the shielding can not be eliminated. Thus, the conventional shielding method can not be employed for such applications where the voltage is determined in terms of an integrated value.
In recent years, in accompanying diversification of the decentralized power supply system, the line current flowing through distribution lines exhibits much complicated behaviors. Under the circumstances, there exists a great demand for enhancement of economical operation and reliability of a 6.6-kV distribution system. Since in this distribution system, the voltage measurement is performed for balanced three-phase alternating current (AC), measurement of lower line voltage (6.6 kV/{square root over (3)}) with reference to the ground is more convenient and simpler when compared with the measurement of high line-to-line voltages (inter-line voltage). Additionally, the measuring circuit should preferably be implemented as a low-current/high-impedance measuring circuit from the viewpoint of miniaturization, low power consumption and safety of the measurement. By way of example, impedance of the measuring circuit should preferably be on the order of Gxcexa9 or xcexcA.
However, the measuring circuit of a high impedance and a low current as mentioned above is susceptible to induction of environmental or ambient noise voltage, making it difficult or practically impossible to ensure the accuracy and the stability as desired. As the voltage source of induced noise, adjacent phase voltages existing in the vicinity of the measured phase may be mentioned. In reality, there exist an increasing number of sources of induction complicated in accompanying increase in the number of distribution lines in urban district(s).
In the light of the state of the art described above, it is an object of the present invention to provide a measuring circuit of high impedance and low current suited for measurement of high voltages such as distribution line voltage which can eliminate induction influence of ambient noise voltage sources, to thereby ensure a high-accuracy voltage measurement with high reliability.
In view of the above and other objects which will become apparent as the description proceeds, it is proposed according to an aspect of the present invention that an outer enclosure type shielding resistance member having a significantly lower resistance than a signal circuit arranged in correspondence to an inter-terminal voltage of a high impedance signal circuit is provided and that a function representing a voltage distribution in the shielding resistance member is matched substantially equal to a function representing distribution of an inter-terminal voltage of the signal circuit. By measuring the line voltage through the medium of the high impedance signal circuit provided with the shielding resistance member, measurement of the line voltage can be accomplished with high accuracy without being affected by the induction.
With the arrangement proposed by the present invention, all the induction currents originating in the noise voltage sources are forced to flow through the shielding resistance member disposed around and in opposition to the signal circuit, whereby the induction current is positively prevented from flowing through the signal circuit. Besides, because the function representing the voltage distribution in the signal circuit is essentially matched with that representing the voltage distribution in the shielding resistance member, the inter-terminal voltages of the signal circuit are essentially equal to the voltages at the corresponding points of the shielding resistance member. Consequently, no current flows into parasitic capacitances making appearance between the signal circuit and the shielding resistance member, which means that the influence of the parasitic capacitance can be made negligible. Thus, there can be realized a high impedance measurement circuit which can ensure high precision and accuracy as well as enhanced stability for the measurement.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.